Vitamin D, also known as calciferol, is a fat-soluble vitamin. Its main physiological function is to promote the growth of bones and teeth; Increase the absorption of calcium in small intestine; Regulate the normal metabolism of calcium and phosphorus.

The vast majority of daily foods do not contain vitamin D, and the foods with more contents are: dried mushrooms, dried white radishes, dried fish, butter, and eggs and milk also contain very little vitamin D. Human skin can synthesize vitamin D under the irradiation of sunlight and ultraviolet rays, so it can also produce vitamin D through sunbathing.